Process for the improvement of fixed electrophotographic printing plates which are water-conductive in the image-free parts



Printing plates which are produced by the electrophotographic process are prepared as follows: lithographic supports that are either naturally hydrophilic or are provided with a hydrophilic covering, e.g., supports made of aluminum or zinc or paper, are provided with a photo-.

conductor surface layer, this electrophotographic material is electrically charged and then exposed by the Contact process via a photographic master or by the reflex process in a document camera. The resultant latent electrostatic image is made visible by dusting over with a resin powder and then fixed, e.g., by heating, so that the resin powder is irremovably anchored to the support. The printing plate is prepared from the resultant electrocopies by treatment With an organic solvent or with an alkaline, neutral or acid aqueous solution. The photoconductive coating is thereby removed from the image-free parts and, after the parts that have been bared have, where necessar, been rendered hydrophilic, the image parts are inked up with greasy ink and printing can them be performed.

The preparation of planographic printing plates by the electrophotograpbic method has also been described, as follows. The image made visible with electroscopic powder-a powder imageis transferred to a lithographic support and the transferred powder is then fixed to the support by heating.

By another method, preferably foils or plates made of aluminum or zinc are coated with a thin layer of anthracene or anthraquinone and then charged, exposed imagewise and developed. The plate is then carefully heated to temperatures at which the developer powder melts and the anthracene volatilizes.

A process which also serves for the electrophotographic production of lithographic plates consists of the following steps. A layer, which becomes electrically conductive when struck by light, and which consists of a photosemiconductor suspended in a binder and is present on a support, is electrostatically charged and then exposed imagewise. The electrostatic image is then developed and fixed and this image is converted into a printing plate by a process which renders the image-free regions of the surface of the support hygroscopic. In a modification of this method, the support is first coated with a hygroscopic substance which, in turn, is coated witha photoconductor layer. When the image has been produced, the photoconductor layer is removed in the image-free parts with a solvent and the hydrophilic coating on the supporting material is bared.

The processes described above have the disadvantage that they result in planographic printing plates having an unduly raised text-a characteristic which is undesirable in an offset printing plate because the text then presses into the rubber blanket of the ofi'set machine and de forms it after a short period. In ofiset printing machines that function without a rubber blanket the very prominent relief, which consists of fixed resin from the resin powder, results in chipping of the image surface because of the brittleness of the resins and, hence, leads to rapid deterioration. Thus, really long runs are not possible.

The present invention relates to a process by which fixed printing plates which have been electrophotographically produced and are hydrophilic on the non-printing 3,!58Aifi Patented Nov. 24, 1964.-

areas are improved and made more suitable for the offset printing process. The process of the invention consists in coating fixed electrophotographically produced printing plates, that are hydrophilic in the image-free parts, with aqueous colloidal agents known per so as printing plate preservatives and then, after the coating has dried, treating the plates with lacquers which contain organic solvents. Advantageously, an oleophilic lacquer consisting of a lacquer base and organic solvents is used for the treatment.

During the lacquer treatment, the fixed powder image is dissolved away in the image areas and the lacquer base combines firmly with the image support, e.g., the supporting metal foil. A layer is formed that is only a few thousandths of a millimeter in thickness, thus fulfilling the conditions of planographic printing. The lacquer protects against mechanical abrasion and enables long runs to be obtained.

In the' process of the invention the procedure is, for example, as follows.

A lithographic metal foil coated with an electrophotographic substance is electrostatically charged and exposed image-wise; the latent image thereon is developed with an electroscopic powder and fixed. The foil is then converted into a printing plate by a process in which the coating is removed or rendered hydrophilic in the imagefree parts. The entire image side of the printing foil is then wiped over with an agent commonly used as a printing plate preservative, e.g., a dilute aqueous solution of gum arabic, carboxymethyl cellulose or alginic acid. After the plate has dried, an appropriate quantity of a suitable lacquer is poured thereon and distributed over the whole surface by means of a cotton pad, sponge or similar material. The lacquer dissolves away the fixed electrophotographic powder image (relief image) andcombines firmly with the metal support, but does not adhere to the gummed, image-free parts. The lacquered printing plate is then dried in the air or in a hot air current or in a drying cupboard and afterwards sprayed down vigorously with Water. The lacquer thereupon flakes olf in the imagefree parts but adheres in the image parts so that an appropriately colored image appears on the printing plate. This can be used for printing in an offset machine.

The lacquer used may consist of various commercially available solutions. Such lacquers are used for the lacquering of lithographic plates that have, for example, been presensitized with light-sensitive diazo solutions or which are coated immediately before use with a lightsensitive chromium-rubber layer or a chromium polyvinyl alcohol layer. It was not known hitherto that electrophotographically prepared printing plates could be converted into true planographic printing plates with these lacquers; lacquers of this type contain a synthetic resin as the lacquer base. Examples of suitable ones are: polymers or interpolymers of vinyl chloride, vinyl acetate, and styrene, with or without the addition of unsaturated acids such as maleic acid, as also alkyl resins, phenol resins, epoxide resins, phthalic acid ester resins, and the like.

Commonly used organic solvents are: water-insoluble aliphatic ketones such as methyl isobutyl ketone, di-isobutyl ketone, ethyl amyl ketone or cyclic ketones such as cyclohexanone or esters of aliphatic acids with aliphatic alcohols such as amyl acetate, butyl acetate, methyl glycol acetate, cyclohexyl acetate or partially hydrogenated aromatic hydrocarbons, such as tetrahydronaphthalene.

A dyestufl dissolved in the solution colors the image parts. Usual dyestuffs are those such as Victoria Blue B (Schultz, Farbstofitabellen, 7th edition, vol. I, p. 347, No. 822), Sudan Black BN (Schultz, Farbstofftabellen, 7th edition, supplementary, volume II, p. 261), Oil Red A (Schultz, Farbstoiftabellen, 7th edition, vol. I, page 365, No. 864), Rhodamine B extra (Schultz, Farbstofftabellen,

1 J9 7th edition, vol. I, p. 365, No. 864), Fast Scarlet (Schultz, Farbstoittabellen, 7th edition, vol. I, p. 185, No. 448), Pure Blue (Schultz, Farbstofftabellen, 7th edition, vol. I, p. 435, No. 1003), and the like.

Plasticizers may be added to the lacquer and these improve film-forming properties. Phthalic acid esters are, for example, suitable.

The invention will be further illustrated by reference to the following specific examples:

Example I An aluminum foil coated with a photoconductive layer, e.g. such as described in Belgian Patent 588,660, is electrostatically charged and exposed under a master. The latent electrostatic image thus produced is developed by means of an electroscopic powder and the powder image is then fixed by heating for a period of irom 20 to 40 seconds to a temperature of 180 C. Subsequently, the foil is converted into a printing plate by removing the photoconductive layer in the image-free areas, thus baring the aluminum support. Now the whole image side of the printing plate is wiped over with a 16 percent aqueous solution of gum arabic, Which deposits firmly in the image free, metallic areas only.

After drying, an appropriate quantity of a lacquer of the following composition is poured onto the printing plate:

P.b.w. Amyl acetate 35 Methylglycol acetate 41 Cyclohexanone Interpolymer containing carboxyl groups 7 .5 Dyestuif Reinblau 0.3

The interpolymer used contains 85 percent by weight of vinyl chloride, 14 percent by weight of vinyl acetate, and 1 percent by weight of maleic acid. It is dissolved in the solvent mixture with agitation and the dyestufi is then added. For Reinblau see: Schultz Farbstotftabellen, 7th edition, vol. I, No. 816.

The lacquer is distributed in sweeping movements over the printing plate. In the image parts, the lacquer dis solves the fixed powder and the photoconductive layer beneath, and combines firmly with the metallic support, thus replacing the image by an identical image consisting of a very thin lacquer layer, but does not adhere to the imagefree, gummed areas. After drying with warm air, the printing plate is vigorously rinsed with water. During this rinsing process, gum arabic and lacquer are peeled from the image-free areas in large flakes, while the lacquer adheres firmly to the image areas, so that a blue image becomes visible on the printing plate. It may be used for making long runs in an offset printing machine.

Example 11 The procedure described in Example I is followed, but the lacquer used consists of the following ingredients:

The novolak resin used is a pure, non-hardenable d phenol resin of a melting range of from 108 to 118 C. For Fettrot A see: Schultz Farbstofftabellen, 7th edition, vol. 1, No. 864.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed:

1. A process for improving an electrophotographic printing plate having, on a support, a fixed image formed by developing a latent electrostatic image with an electroscopic material, the plate being hydrophilic in the imagefree areas, which comprises coating the plate with an aqueous coloidal agent, which colloidal agent adheres to the non-image areas, drying, treating the plate with a lacquer containing an organic solvent, whereby only the image areas are removed and the said lacquer adheres to the said support in the image areas, and treating the plate with water whereby the coating is removed only in the non-image areas.

2. A process according to claim 1 in which the aqueous colloidal agent is selected from the group consisting of gum arabic, carboxymethyl cellulose, and alginic acid.

3. A process according to claim 1 in which the organic solvent is selected from the group consisting of a ketone, an ester, and a partially hydrogenated aromatic hydrocarbon. V

4. A process according to claim 1 in which the organic solvent is tetrahydronaphthalene.

5. A process according to claim 1 in which the organic solvent is cyclohexanone.

6. A process according to claim 1 in which the organic solvent is amyl acetate.

7. A process according to claim 1 in which the organic solvent is methylglycol acetate.

8. A process according to claim 1 in which the lacquer includes a dyestutf.

9. A process according to claim 1 in which the lacquer includes a synthetic resin.

10. A process according to claim 1 in which the lacquer includes a synthetic resin selected from the group con sisting of a polymer of vinyl chloride; a copolymer of vinyl chloride, vinyl acetate and styrene; a copolymer of vinyl chloride, vinyl acetate, styrene, and maleic acid; a phenolic resin; an aldehyde resin; an epoxy resin; and an alkyd resin.

References Cited in the file of this patent UNITED STATES PATENTS 2,754,279 Hall July 10, 1956 2,865,873 Hodgins Dec. 23, 1958 3,019,106 Adams Jan. 30, 1962 3,121,009 Giaimo Feb. 11, 1964 FOREIGN PATENTS 210,374 Australia Sept. 12, 1957 28,392 Great Britain Nov. 2, 1905 OTHER REFERENCES Strauss: The Lithographers Manual, vol. 1, Walt win Publ. Co., New York, January 1958; copy in- Sci. Lib. Card Cat. No. TR/940/S6/19S8; pages 10:26-10:29 and 10:42 relied on. 

1. A PROCESS FOR IMPROVING AN ELECTROPHOTOGRAPHIC PRINTING PLATE HAVING, ON A SUPPORT, A FIXED IMAGE FORMED BY DEVELOPING A LATENT ELECTROSTATIC IMAGE WITH AN ELECTROSCOPIC MATERIAL, THE PLATE BEING HYDROPHILIC IN THE IMAGEFREE AREAS, WHICH COMPRISES COATING THE PLATE WITH AN AQUEOUS COLOIDAL AGENT, WHICH COLLOIDAL AGENT ADHERES TO THE NON-IMAGE AREAS, DRYING, TREATING THE PLATE WITH A LACQUER CONTAINING AN ORGANIC SOLVENT, WHEREBY ONLY THE IMAGE AREAS ARE REMOVED AND THE SAID LACQUER ADHERES TO THE SAID SUPPORT IN THE IMAGE AREAS, AND TREATING THE PLATE WITH WATER WHEREBY THE COATING IS REMOVED ONLY IN THE NON-IMAGE AREAS. 